[OPENMP][NVPTX]Improved several standard OpenMP functions, NFC.

Summary:
Used parallelLevel[] counter to simplify and improve implementation of
the existing standard OpenMP functions. Functions are tested already in
several tests, the patch is NFC.

Reviewers: grokos, gtbercea, kkwli0

Subscribers: guansong, jdoerfert, caomhin, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D61459

llvm-svn: 359892